Numerous aerial devices have heretofore been provided for elevating workmen to overhead locations for the performance of various work functions, but heretofore all have suffered from a certain lack of maneuverability, strength, freedom from the risk of electrical shock, and lifting power. Accordingly, one important object of the present invention is to provide a man-lifting aerial device which has the power, maneuverability and strength of a heavy-duty, articulating crane, yet is capable of safely maintaining the workmen carried by the bucket of the device electrically insulated from ground potential at all times so that, if desired, the workmen can safely service, maintain and install high voltage, overhead electrical transmission lines and the like, even in awkward, hard-to-reach places.
Pursuant to the foregoing, the present invention contemplates attaching a special, electrically insulated, telescoping third section onto the outer end of two, articulating sections of a crane so that the man-lifting bucket carried by the outer tip end of the insulated section can be maneuvered to virtually any desired location within reach of the crane yet is safety insulated from structures associated with the crane which would otherwise tend to provide an electrically conductive path to ground potential. More specifically, the telescoping third section has three tubular, telescoping booms which are extended and retracted by a pair of internally housed, hydraulic piston and cylinder units mechanically interconnected and hydraulically coupled in such a way that the outermost fiberglass boom of the section always extends first and retracts last, thereby maintaining the lifting bucket at the outer end of the fiberglass boom remote from the two metal booms of the third section during significant periods of use of the machine. A radio transmitter located in the lifting bucket enables the operator to control all maneuvering functions of the crane without electrically conductive cables, wires or other structures which have in the past been necessary from such buckets. Special structural provisions are made internally of the telescoping third section of the crane to avoid undue bending and stress in an unusually long piston rod associated with one hydraulic cylinder of the section.